The subject matter disclosed herein relates to X-ray tubes, and in particular to emitters for use in X-ray tubes.
Presently available medical X-ray tubes typically include a cathode assembly having an emitter and a cup. The cathode assembly is oriented to face an X-ray tube anode, or target, which is typically a planar metal or composite structure. The space within the X-ray tube between the cathode and anode is evacuated.
X-ray tubes typically include an electron source, such as a cathode, that releases electrons at high acceleration. Some of the released electrons may impact a target anode. The collision of the electrons with the target anode produces X-rays, which may be used in a variety of medical devices such as computed tomography (CT) imaging systems, X-ray scanners, and so forth.
To improve the useful life of the emitters used to generate the electron beams and thus the useful life of the X-ray tubes, a flat surface emitter (or a ‘flat emitter’) may be positioned within the cathode cup with the flat surface positioned orthogonal to the anode, such as that disclosed in U.S. Pat. No. 8,831,178, incorporated herein by reference in its entirety. In the '178 patent a flat emitter with a rectangular emission area is firmed with a very thin material having electrodes attached thereto, which can be significantly less costly to manufacture compared to conventionally wound (cylindrical or non-cylindrical) filaments and may have a relaxed placement tolerance when compared to a conventionally wound filament.
X-ray tubes having cathodes with flat emitters can control the flow of electrons from the emitter to the target using a grid electrode. The electron emission originating from the surface of a thermoionic electron emitter, the flat emitter, strongly depends on the “pulling” electric field generated by the X-ray tube's anode. For enabling fast on/off switching of the tube, it is known from the relevant prior art that X-ray tubes of the rotary-anode type may be equipped with a grid electrode placed in front of the electron emitter. To shut off the electron beam completely, a bias voltage is applied to the grid electrode which generates a repelling field and is usually given by the absolute value of the potential difference between the electron emitter and the grid electrode. The resulting electric field at the emitter surface is the sum of the grid and the anode generated field. If the total field is repelling on all locations on the electron emitter, electron emission is completely cut off.
Additionally, in X-ray tubes employing a flat filament/emitter and focal spot control via electrostatic focusing, such as disclosed in co-owned U.S. Pat. No. 8,401,151, entitled “X-Ray Tube For Microsecond X-Ray Intensity Switching” the entirety of which is expressly incorporated by reference herein for all purposes, the electron beam drifts a distance of several centimeters past the anode in electric field free region before reaching the target. Due to the increased travel distance more residual gas ions are produced.
However, in all X-ray tubes an amount of residual gas is present within the tube as a result of the manufacturing processes for the tubes. When electrons generated by the emitters and drawn towards the anode strike the residual gas, the gas becomes ionized. As this ion charge is opposite that of the electrons generated by the emitter and ions are much heavier than electrons, the ions are drawn to the center 300 of the emitter 1000 where these ions strike the emitter 1000 causing damage to the emitter surface through sputtering and/or local heating as shown in FIG. 3. Over time, this damage accumulates and can completely break or sever the ribbon of material forming the emitter 1000, thereby severing the circuit for current flow through the emitter and rendering the X-ray tube inoperative.
To limit the ion bombardment of prior art emitters, various types of ion barriers are utilized. These ion barriers are disposed upstream from the emitter and operate to draw the ions in or onto the barriers or inhibit ions to travel past the barriers. However, while effective in preventing ions from bombarding the emitters, these ion barriers create significant additional complexity and, expense in the construction of the X-ray tube.
Hence it is desirable to provide an X-ray tube with an emitter which can effectively limit the damage caused to the emitter as a result of ion bombardment, thereby increasing the useful life of the emitter and the X-ray tube without significantly increasing the complexity or cost of the construction of the X-ray tube.